docs: correct Image Generation docs per review of PR #335

[roomote]

This PR applies corrections identified in internal review of PR #335:

• Replace "choose a save path" with "specify or accept suggested path" to match current UX (no picker)
• Add note clarifying that image generation runs via Gemini 2.5 Flash Image Preview on OpenRouter, regardless of chat model; chat model just needs tool use
• Keep model list and formats consistent with implementation

Refs: #335

---

Important

Update image-generation.md to reflect UX changes and clarify model usage for image generation.

• Documentation Updates:
  • Replace "choose a save path" with "specify or accept suggested path" in image-generation.md to match current UX.
  • Add note in image-generation.md clarifying image generation runs via Gemini 2.5 Flash Image Preview on OpenRouter, regardless of chat model.
  • Ensure model list and formats in image-generation.md are consistent with implementation.

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Project	Deployment	Preview	Comments	Updated (UTC)
roo-code-docs	Ready	Preview	Comment	Sep 7, 2025 5:51pm

[hannesrudolph]

@roomote-agent apply them to the PR... don't make a new PR. 🤦 close this one please.